The detrimental effects of the burning of fossil fuels on the environment are becoming more and more of a concern and have spurred great interest in alternative energy sources. While progress is being made with solar, wind, nuclear, geothermal, and other energy sources, it is quite clear that the widespread availability of economical alternate energy sources, in particular for high energy use applications, remains an elusive target. In the meantime, fossil fuels are forecast to dominate the energy market for the foreseeable future. Among the fossil fuels, natural gas is the cleanest burning and therefore the clear choice for energy production. There is, therefore, a movement afoot to supplement or supplant, as much as possible, other fossil fuels such as coal and petroleum with natural gas as the world becomes more conscious of the environmental repercussions of burning fossil fuels. Unfortunately, much of world's natural gas deposits exist in remote, difficult to access regions of the planet. Terrain and geopolitical factors render it extremely difficult to reliably and economically extract the natural gas from these regions. The use of pipelines and overland transport has been evaluated, in some instances attempted, and found to be uneconomical. Interestingly, a large portion of the earth's remote natural gas reserves is located in relatively close proximity to the oceans and other bodies of water having ready access to the oceans. Thus, marine transport of natural gas from the remote locations would appear to be an obvious solution. The problem with marine transport of natural gas lies largely in the economics. Ocean-going vessels can carry just so much laden weight and the cost of shipping by sea reflects this fact, the cost being calculated on the total weight being shipped, that is, the weight of the product plus the weight of the container vessel in which the product is being shipped. If the net weight of the product is low compared to the tare weight of the shipping container, the cost of shipping per unit mass of product becomes prohibitive. This is particularly true of the transport of compressed fluids, which conventionally are transported in steel cylinders that are extremely heavy compared to weight of contained fluid. This problem has been ameliorated somewhat by the advent of Type III and Type IV pressure vessels. Type III pressure vessels are comprised of a relatively thin metal liner that is wound with a filamentous composite wrap, which results in a vessel with the strength of a steel vessel at a substantial saving in overall vessel weight. Type IV pressure vessels comprise a polymeric liner that is likewise wrapped with a composite filamentous material. Type IV pressure vessels are the lightest of all the presently approved pressure vessels. The use of Type III and Type IV vessels coupled with the trend to make these vessels very large—cylindrical vessels 18 meters in length and 2.5-3.0 meters in diameter are currently being fabricated and vessel 30 or more meters in length and 6 or more meters in diameter are contemplated—has resulted in a major step forward in optimizing the economics of ocean transport of compressed fluids.
The trend to make Type III and Type IV pressure vessels very large carries with it a unique set of challenges, one of which relates to the conditions under which prepolymer formulations appropriate for use in such pressure vessels can be cured to form the final product, be it a pressure vessel liner, a composite over-wrap, a composite dome on a cylindrical pressure vessel or a composite boss for fitting a pressure vessel to external paraphernalia for loading and unloading fluids from the pressure vessel. That is, polymers suitable for use in the manufacture of a pressure vessel must have the strength to withstand high operating pressures, must have adequate impact resistance to minimize chances of catastrophic failure on inadvertent impact, must be essentially impermeable and inert to compressed fluids contained in the vessels and should have as broad a range as possible of operating temperatures under which the vessel can be safely used.
A currently preferred polymeric material that exhibits such performance characteristics is high density polyethylene (HDPE). The problem with HDPE is the cure conditions that must be used to form the polymeric end product. That is, HDPE must generally be cured at temperatures in excess of 450° F. to obtain pressure vessel liners and composite over-wraps, the two uses for which it is currently in use. While this is not a great problem for the manufacture of small vessels, when the size of the construct to be cured is increased to the dimensions contemplated for marine transport of fluids such a compressed natural gas (CNG)—pressure vessels 3 meter in diameter and 18 meters in length are currently being produced and vessel over 6 meters in diameter and over 30 meters in length are contemplated, the sheer magnitude of the required curing facility that can contain the construct in a controlled high temperature environment and the cost of operation become prohibitive.
What is needed is a high performance, variable viscosity, variable cure temperature (in particular low temperature cure) prepolymer formulation for use in the fabrication of polymeric pressure vessels. The present invention provides such a prepolymer formulation.